Various tools for treating the root canal of a tooth exist, including, for example, a reamer and a file that cut and form a root canal, a compactor, a filler, a spreader, and a plugger that fill the root canal with thermoplastic resin; a cleanser that removes remainders in the root canal; and a broach that fills the root canal with cotton. The above-described reamer and file is constructed of a slender tapered rod having a work portion provided with a cutting blade or piercing protrusion appropriate for a treatment object or a member having a work portion formed by forming a tapered rod in a spiral shape. Moreover, the tools described above comprised of a handle or a grip integrated with the end of the member, so that the tool may be gripped or operated by a dentist, or is constructed of a shank, so that it may be gripped by a chuck of a hand piece or the like, or is directly operated by the dentist.
The root canal is excessively thin and has a variety of shapes and sizes, and varies between persons. For this reason, tools of many different sizes are needed, even for the same kind of root canal treatment tools. For example, in the case of cutting and forming a root canal by use of a reamer, the reamer is required to be deformed along the shape of the root canal, that is, to have appropriate elasticity so that it should not hurt the surrounding root canal. Japanese Patent No. 3375765 discloses a root canal treatment tool having extremely high elasticity and a shape restoration property as described above. This technology relates to a root canal treatment tool having a work portion formed and manufactured by applying a removing work to a rod-shaped raw material that is subjected to shape memory heat treatment, and has a superelastic characteristic while holding it under a memory treatment temperature.
In the above-mentioned root canal treatment tool, the rod having the work portion formed thereon is flexibly deformed in response to an external force applied thereto, and is quickly restored to an original shape when the external force is removed. For this reason, this tool can conform well to the shape of the root canal, and form the root canal with high accuracy.
In the root canal treatment tool relating to the Japanese Patent No. 3375765, the work portion has the superelastic characteristic uniformly along the whole length and, hence, when the work portion is bent, a tip portion of a free end also has an action of returning to an original shape. Accordingly, when the tip portion is inserted into the root canal for the treatment of the root canal and is bent, a repulsive force is generated to act on the wall of the root canal. Thus, there is an undesirable possibility that, in the vicinity of a root sharp mouth (tip of the work portion), the work portion might cut the outside of a bent portion of the root canal more heavily, and might cut the inside in the center of the bent portion more heavily, so as to penetrate the root canal.
Moreover, since the root canal treatment tool is rotated in a state where the work portion is bent almost along the whole length from the tip at the time of forming the root canal, a repeated bending stress is applied to the work portion. This increases the possibility of breaking a slender tip portion of the work portion. For this reason, although the root canal treatment tool relating to the Japanese Patent No. 3375765 has sufficiently high durability as compared with a root canal treatment tool made of stainless steel, it lacks sufficient durability.